Coaxial tubing having improved spacer means



T- STEARNS 3,490,496

COAXIAL-TUBING HAVING IMPROVED SPACER MEANS Jan. 20,- 1970 Filed Jan. 15, 1968 FIG 6 United States Patent 3,490,496 COAXIAL TUBING HAVING IMPROVED SPACER MEANS Thornton Stearns, Winchester, Mass, assignor to Vacuum Barrier Corporation, Woburn, Mass., a corporation of Delaware Filed Jan. 15, 1968, Ser. No. 697,747 Int. Cl. F161 9/18 US. Cl. 138-112 14 Claims ABSTRACT OF THE DISCLOSURE Coaxial tubing, as for cryogenic transfer lines, has a spacer for inner and outer preferably spirally corrugated tubes, the spacer being a plastic ribbon spirally twisted about its own axis and also wound helically around the inner tube to contact the inner tube at spaced intervals only and with its opposite continuous edges contacting the inner tube at said intervals in alternation, as for example, at every 90 of its revolution around the inner tube, thus simultaneously presenting its other continuous edge facing radially outwardly in alternation at every 90 interval so that the ribbon extends at every 90 interval transversely of the annular space between the two tubes for maintaining the two tubes in concentric relation. In modified forms, layers of insulating wrap may be interposed either between the spacing and the outer tube or around the inner tube radially inwardly of the spacer of both, and the ribbon, instead of being of uniform width, may have its edges notched to conform better to the contour of the inner tube and give better stability to the spacer.

This invention relates to coaxial tubing and more particularly to flexible transfer lines for cryogenic liquids having concentric spaced tubes providing an interveningannular insulating space which may contain insulation or be evacuated or both, though the tubing is also useful for other purposes such as electrical cable.

Coaxial tubing, whether rigid or having a degree of flexibility, requires some structure between the tubes which will maintain the tubes in generally concentric relation.

Unfortunately, any solid rigidifying structure bridging the space between the tubes and provided for this purpose forms a path for thermal leakage (or dielectric losses) and the greater the minimum cross-sectional dimension of the structure and the greater the contact area thereof with the tube walls, the greater the losses. Additionally, the provision of bridging rigidifying structure can complicate evacuation of the annular insulating space after fabrication of the tubing.

One form of spacer means that has found use is a plastic square or rectangular ribbon which is wound spirally around the inner tube and has a thickness approximating the minimum radial dimension of the annular insulating space and a cross-width dimension sufficient to prevent its collapse. Such ribbons are particularly used where the outer tube is continuously formed around the ribbon-wound inner tube.

In accordance with this invention, the losses encountered in such prior structures as well as the amount of ribbon material required per unit length of coaxial tubing are substantially reduced by the expedient of twisting a flat ribbon about its own axis before or as it is laid into the inner tube so that, for example, a 180 twist is made in the ribbon for each quarter revolution of helical winding around the inner tube, with the consequence that a ribbon of uniform width approximating the maximum radial dimension of the annular space between the tubes will at most contact the tubes edgewise only at spaced intervals with one continuous edge of the ribbon first contacting the inner tube and then contacting or at least facing the outer tube at the next twist and thus alternating so on down the length of the tubing. Meanwhile, the opposite edge of the ribbon alternately also contacts first the inner and then faces or contacts the outer tube down the length of the tubing at intervals spaced between the intervals of contact of the first edge and usually offset circumferentially around the inner tube from the points of contact of the one edge with the inner tube. For the purpose of the description herein given, the inner tube may be smooth or corrugated with or without surrounding insulation wrap. If corrugated, it may also include a surrounding reinforcing wire braid. Similarly the outer tube may be smooth or corrugated with or without an inner lining of insulation.

Assemblies of this invention provide substantial advantages which will hereinafter be described after a better understanding of the structures is provided by reference to typical embodiments of the invention as shown in the accompanying drawings wherein:

FIG. 1 is a vertical cross-sectional view through a typical embodiment of the invention; and

FIG. 2 is a similar cross-sectional view taken with the tubing of FIG. 1 rotated end to end 180;

FIG. 3 is a plan view of a modified form of ribbon that may be used in lieu of the ribbon shown in FIGS. 1 and 2;

FIG. 4 is a similar plan view of a still further modified form of ribbon;

FIG. 5 is a cross-sectional view showing the ribbon of FIG. 4 in the structure of FIG. 1;

FIG. 6 is a vertical cross-sectional view showing a modified form of assembly; and

FIG. 7 is a cross-sectional view of an assembly like that of FIG. 6 but omitting insulating wrap on the inner tube.

In FIGS 1 and 2 of the drawings, 10 indicates a spirally corrugated inner tube and 12 a spirally corrugated outer tube. Such tubing is commonly made of copper and has a degree of flexibility on a fairly large radius. Helically wound around the inner tube 10 is a length of flat metal or plastic ribbon 20, preferably polytetrafluoroethylene, of uniform width with a pitch of approximately 45 relative to the axis of the tubing.

The ribbon 20 is twisted 180 about its own axis every of its revolution around the tube 10 so that one or the other of its opposite edges contacts the inner tube edgewise once every 90 of each revolution.

Thus edge 21 of ribbon 20 faces radially outwardly at points A, C, E, G, I, K and M, whereas it faces radially inwardly and contacts the inner tube 10 at points B, D, F,H,J,Land N.

The opposite edge 22 of the ribbon accordingly faces radially inwardly and contacts tube 10 at points A, C, E, G, I, K and M; and faces radially outwardly at the intervening points B, D, F, H, I, L and N.

Also it will be noted that where either one of the edges faces radially inwardly, it tends to seat itself in one of the troughs 30 intervening the crests 32 on the inner tube 10. This tends to happen at the points B, F, J and N in FIG. 1 and at D, H, and L in FIG. 2 of the drawings, although for the purposes of clarity, the drawing shows the ribbon in a more nearly symmetrical spiral at these points.

Normally the ribbon is of such width that it may, but does not necessarily, contact the outer tube at all the twist points, but the ribbon does form a spacer which will maintain the two tubes in concentric spaced configuration.

The pitch of the ribbon may be something other than 45 and the ribbon may be twisted at closer os longer intervals along its length than those shown in the drawings, depending upon how much support between the two tubes is necessaryor desirable. In any event, as shown in the drawings, there never is any bridging contact between the two tubes so far as the'spacer is concerned except at the points A-N inclusive, where the ribbon is radially oriented, with the increments of the ribbon between these points always laying spaced from the outer tube due to the twisting configuration of the ribbon.

For example, where the outer tube has an inside diameter of 1.245 inches and the inner tube has an outside diameter of .795 inch, the ribbon may be .285 inch wide and .04 inch thick. In cryogenic use, with a vacuum better than 10 torr, heat leakage has been reduced to only 1.2 B.t.u. per hour per linear foot when measured between room temperature and 320 F. For larger lines the tape width may run up to .4 inch and a greater thickness may be used running up to .1 inch or more depending upon the radial dimension between the tubes.

Application of the structure of the invention is not confined to corrugated tubing but is useful also with straight rigid tubes. The inner tube when corrugated may include an outer tubular braided reinforcing wrap 50, as shown in FIG. 6, such is is often now used with other spacer constructions for increasing the pressure rating and as shown for instance in US. Patent No. 3,240,234.

FIG. 3 shows a modified ribbon 20a, the same as that used in FIGS. 1 and 2 except that its edges are provided with cut-outs or notches 40 in staggered relation down the length of the ribbon and spaced at such intervals that the notches will, in theassembled line, engage the inner tube at the points where the twisting ribbon will extend approximately radially across the annular space between the tubes. The notches may have a radius slightly greater than that of the inner tube because of the angle of contact as shown in FIG. 5. The notched configuration does afford a gain in stability in the ribbon because of the more conforming contour of the ribbon with the surface of the tube at the contacting points.

FIG. 4 shows a still further modified form of ribbon 20b wherein the notches-40 instead of being staggered down the length of then'bbon appear periodically on opposite sides of the ribbon, thus making it immaterial which particular side of the ribbon is presented to the inner tube since both sides are notched. FIG. 5 shows thei ribbon of FIG. 4 positioned in the tubing of FIGS. 1 an 2.

In FIG. 6 the diameter of outer tube 12a has also been enlarged so that helically or spirally loosely wound multiple layers of insulation of the type, for example, described in US. Patent No. 3,240,234, and comprising alternate layers of aluminum coated plastic film with fiberglass sheetingin a loose Wrap may be made a part of the inner tube. The reinforced and insulation wrapped inner tube is then wound with the twisted ribbon 20, 20a or 20b. Thereafter another layer of insulation is wrapped over the ribbon and then the assembly is enclosed in the outer tube 12a. The insulation 52 and 54 is thus compressed only at the points of radial orientation of the ribbon, leaving the optimal density of the wrapping, as explained in US. Patent No. 3,236,406, throughout the remainder of the annular space between the tubing.

FIG. 7 shows loose wrap multiple layer insulation interposed only outside of the wound ribbon 20, again with compression of the layers occurring substantially only at the points of radial orientation of the ribbon. As is obvious, instead of omitting, as in FIG. 7, the inner tube insulation layer, the outer tube insulating liner 54 may be omitted while retaining the inner tube insulation wrap 52 with or without the reinforcing tubular wire braid 50. There is thus provided a spacer means in all forms which may be readily placed between the two tubes during continuous forming operations using a minimum of material for the spacer. Under certain circumstances, particularly in larger sizes, two or more separate twisted ribbons may be wound on the inner tube in spaced relation both having the same pitch and having either the same or difierent twist intervals.

The construction also makes it easier to draw a vacuum from the passage between the two tubes, whether containing insulation layers or not, because of the minimum obstruction by the spacer, less than is encountered with untwisted ribbon. Moreover, the required rigidity against collapse is afforded, because of the twists, by a much thinner ribbon than that which could be used in the absence of the twists of the ribbon about its own axis.

What is claimed is:

1. Coaxial tubing comprising:

inner and outer concentrically arranged tubes, and

spacing means extending longitudinally of said tubing between said tubes and comprising a continuous fiat ribbon, said ribbon being twisted about its own axis to present opposite continuous edges which contact said inner tube alternately at spaced intervals along said edges with said flat ribbon extending radially of said tubing only at the same intervals.

2. Coaxial tubing as claimed in claim 1, wherein said ribbon is of uniform width.

3. Coaxial tubing as claimed in claim 1, wherein said ribbon is a plastic ribbon.

4. Coaxial tubing as claimed in claim 1, wherein said edges face radially outwardly between their successive adjacent points of contact with said inner tube.

5. Coaxial tubing as claimed in claim 1, wherein edges of said ribbon have notches where said edges contact said inner tube to aid stabilizing said ribbon.

6. Coaxial tubing as claimed in claim 3, wherein said ribbon has additional notches in its edges opposite the notches at the inner tube contacting intervals.

7. Coaxial tubing as claimed in claim 1, wherein said inner tube includes an outer loose wrap of multiple layers of insulation compressed only at said spaced intervals.

8. Coaxial tubing as claimed in claim 1, wherein a loose wrap of multiple layers of insulation surround said ribbon, said wrap being compressed only where said edges face radially outwardly.

9. Coaxial tubing as claimed in claim 8, wherein said inner tube includes an outer wrap of loose multiple layers of insulation compressed only at said spaced intervals.

-10. Coaxial tubing as claimed in claim 1, wherein said twisted ribbon is helically wound around said inner tube.

11. Coaxial tubing as claimed in claim 10, wherein said ribbon is twisted per each quarter revolution of its helical winding around said inner tube.

12. Coaxial tubing as claimed in claim 10, wherein the pitch of said helical winding is about 45 relative to the axis of said tubing.

13. Coaxial tubing as claimed in claim 10, wherein said ribbon extends radially of said tubing only at circumferentially spaced intervals along its helical winding.

14. Coaxial tubing as claimed in claim 13, wherein said inner tube has a spirally corrugated configuration to form an outer surface having external crests and intervening grooves and said radially extending portions of said ribbon are seated in the grooves between the crests in the walls of said inner tube.

References Cited UNITED STATES PATENTS Re. 8,745 6/1879 Merriam 138-114 1,959,367 5/1934 Kennedye 138-148 2,714,395 8/1955 Epstein 138-413 3,332,446 7/1967 Mann 138148 FOREIGN PATENTS 1,008,040 10/ 1965 Great Britain.

HERBERT F. ROSS, Primary Examiner US. Cl. X.R. 138-114 

